EB tries to simulate history as much as it can within the constraints of the engine. As such, it makes sense that Rome can obtain heavy infantry cheaper and easier than Armenia. Rome has a tradition citizen heavy infantry while Armenia drew her main warriors from the nobility, who were horsemen. On the other hand, Rome's cavalry is generally bad and overpriced while the Armenians have access to some of the best cavalry units in the game.
That's not to say the mod is entirely realistic. Historically, the Seleucids would never focus on Armenia like they do in the game, because they would be distracted by other threads, but the A.I. is programmed to hate the player. As result, once the war starts it will be almost impossible to stop it, and the Seleucids can and will throw army after army at you. Hence Armenia is marked as nigh-impossible.
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